I think the answer Layna gave you would be the easiest and maybe the preferable way. But sometimes it is not possible to give back the puppy. So for this case, you have two options:

1. Seperate them forever or until you can sell the puppy.
2. Seperate them until your training with your boxer is so successful that you can stop him to bite without a leash and without intervening physically.

The seperation is recommended, because every attack from your boxer has some psychological impact to your puppy. In the worst case he will start to bite every other dog when he is older or having at least the same problem with puppies.

In case you want to train your boxer, keep in mind that looking for a good dog trainer is not a shame, but highly recommended in a situation like yours. Look for someone who can teach you in personal. Tips given on the internet can never be so specially geared to you and your dog, because we have never seen you in real.

How to train:

1. Train commands like "come" and "sit" in every situation. Start with small steps at home and increase the external stimuli. You have to be able to have your boxer under control in any situation.

2. Start to bring your boxer and the puppy together. Stay always calm. Call your boxer to sit next to you when you notice that he is aggressive, but before he starts to attack the puppy. Start with just a view minutes training time and increase the time they are together slowly.

3. If you keep both dogs, take care about giving both the same attention. Play with both and train with both.

Having two dogs is always hard. The effort you have to spend for training is more than doubled, because dogs tend to adopt bad behavour of the other, so the owner will always have more trouble to train each of them. But well trained dogs in a pack are really nice. So think about it. Can you give your dogs these kind of intensive time and training until the problem with your boxer is solved and afterwards?

It will be a hard and long way, if you keep the puppy. But even if you decide to give the puppy away, you should train with your boxer.

I think the answer Layna gave you would be the easiest and maybe the preferable way. But sometimes it is not possible to give back the puppy. So for this case, you have two options:

1. Separate them forever or until you can sell the puppy.
2. Separate them until your training with your boxer is so successful that you can stop him biting without a leash and without intervening physically.

The separation is recommended, because every attack from your boxer has some psychological impact to your puppy. In the worst case he will start to bite every other dog when he is older or have at least the same problem with puppies.

In case you want to train your boxer, keep in mind that looking for a good dog trainer is not a shame, but highly recommended in a situation like yours. Look for someone who can teach you in person. Tips given on the Internet can never be so specially geared to you and your dog, because we have never seen you for real.

How to train:

1. Train commands like "come" and "sit" in every situation. Start with small steps at home and increase the external stimuli. You have to be able to have your boxer under control in any situation.

2. Start to bring your boxer and the puppy together. Stay always calm. Call your boxer to sit next to you when you notice that he is aggressive, but before he starts to attack the puppy. Start with just a few minutes' training time and increase the time they are together slowly.

3. If you keep both dogs, take care to give both the same attention. Play with both and train with both.

Having two dogs is always hard. The effort you have to spend for training is more than doubled, because dogs tend to adopt bad behavour of the other, so the owner will always have more trouble to train each of them. But well trained dogs in a pack are really nice. So think about it. Can you give your dogs these kind of intensive time and training until the problem with your boxer is solved and afterwards?

It will be a hard and long way, if you keep the puppy. But even if you decide to give the puppy away, you should train with your boxer.